The ability to measure the total pressure within a sealed container, as well as the concentration of a particular gas in the container, is very important in several fields, ranging from the drug industry to food production. In the wine industry, for example, the ability to take a precise measurement of the concentration and pressure of carbon dioxide inside bottles (but without damaging either the bottle or its contents in any way) has several important applications: it is useful for distinguishing between champagne and sparkling wine (which have different allowable levels of carbonation); for monitoring the state of conservation of a single bottle; and even for fine-tuning the equipment used in the bottling process. Similarly, in the pharmaceutical and food packaging sectors (e.g. lyophilized components) the determination of the oxygen content is critical as an indicator of the conservation state of the product and of the integrity of the container.
At present, the methods commonly used for these kinds of pressure measurements include electronic pressure or vacuum gauges; pressure-measuring cells based on ceramic technology; piezoresistive sensors, etc. However, all of these methods share the disadvantage of being invasive, because in order to measure the gas present in a given container they need to extract a sample, and thereby spoil the integrity of the container's cork or seal. Moreover these methods are difficult to integrate as on-line controls in the production process.
For sparkling wine, for example, the current standard instrument for measuring above-atmospheric pressure is the aphrometer, which functions by piercing the bottle's cork or seal and measuring the total pressure inside. Unfortunately, the aphrometer is not able to measure the concentration of individual gases (such as carbon dioxide) or the partial pressure of the gases that comprise the total atmosphere within the bottle. This is an important shortcoming, because regulations regarding certain kinds of wine production distinguish between the partial pressure from carbon dioxide and the total pressure within the bottle (that can be due also to the contribution of other gases like nitrogen and oxygen). As such, it is clear that the aphrometer is not an adequate instrument.
In recent years, some novel measuring techniques for the detection of oxygen have been developed based on optical or opto-acoustic methods such as absorption spectroscopy in the modality TDLAS (Tunable Diode Laser Absorption Spectroscopy) or FMS (Frequency Modulation Spectroscopy). These systems have been studied for applications in the pharmaceutical sector, where a production-line control is required on any single piece. However, these systems have been limited to the detection of oxygen, and need reference container for calibration. A typical system is described by Veale et al. (U.S. Pat. No. 6,639,678 B1) which shows a system using derivative spectroscopy applied to the detection of pressure inside transparent containers and with predetermined background detection and calibration.
In contrast, we propose an apparatus to measure the pressure of the gas inside a generic bottle made of glass or other partially optically transparent materials, where the optical properties, such as absorption and scattering, are completely unknown. In this case the background absorption must be reconstructed. A novel calculation method that allows the self-referenced determination of the measured quantity, being it the pressure or the concentration, is disclosed. Our invention offers a method for measuring in real time (i.e. during the bottling filling phase and the conservation period) the pressure and the concentration inside a sealed container without reference container for calibration. Furthermore, this is accomplished by means of a measuring system that is simple, easy to operate, and usable both on the bottling lines as well during after-production controls, by using a spectroscopic method well known as the TDLAS spectroscopy.
These and others purposes are achieved by the invention with a measuring method and implementation whose main characteristics are described in the claims paragraph.